I've done several Panama Canal cruises all in April different years.
The weather was very warm and hot. Mostly dry except for some occasional Caribbean rain that lasted 10 minutes. Then it dried up and you never knew it rained.
Pack plenty of sunscreen for when you are on deck watching the ship do the Canal.
I recommend watching some of it on deck and the rest from inside with the air conditioning.
Definitely pack walking shoes, shorts or skorts for the deck or ports and plenty of short sleeve tops.
Formal nights are formal attire if you are eating in the dining room. Celebrity enforces this policy. You can go more casual if you eat at the alternative restaurants.
Pack plenty of film. There is lots to see on this itinerary.
Please write me if you have any additional questions.
Have a great and memorable time,